Techniques of light reflection have been used to study the intrinsic properties of skin, (such as the adsorption and the scattering) coefficients, and mathematical models have been developed to take into account these properties. These studies were performed mainly to determine skin chromophores due to changes in the skin structure and the effects of pigments and hemoglobin on the reflection characteristics of the skin. Specific areas include skin erythema due to ultraviolet light exposure, and mechanical compression of the skin. Another technique, laser speckle and holography has been used to measure small displacements of skin. However, this interferometric technique has some severe technical difficulties for in vivo use due to motion of the subject. Also a laser diffraction apparatus has been designed to measure the sarcomere length of the muscles in-vivo, which is useful for tendon transfer procedures. The diffraction patterns of the striated muscle fibril due to Z dark lines have been utilized in this technique. This method will not work for skin or other soft tissues due to the lack of fine natural grating lines for laser diffraction.
The optical properties of skin are related to its structure and its chemical composition such that when a beam of light reaches the skin surface, part of it will be reflected by the surface directly, while the rest will be refracted and transmitted into the skin. The direct reflection of light by a generally smooth surface, where the angle of reflected light is equal to angle of incident light is called specular reflection and is related only to the refractive index change between the air and reflecting medium. By nature, most of the reflection of light from skin is diffuse rather than specular. The intensity and angular distribution of diffusion reflection is determined by transmission and scattering properties of the skin tissue. After diffuse reflection from the top layer of the skin, a portion of the light transmitted through the top layer and into the skin will be scattered and absorbed by the skin tissue. After multiple scattering and reflection from the different layers of skin, some of the transmitted light will re-emerge through the air-stratum interface into the air as part of the total reflected light intensity.
Soft tissue stretch is commonly measured by marking the tissue on its surface using a pen or colored tape and recording the changes in marker position with stretch. The recording device can be photographic film or video. In this method a large distance between the marked lines on the skin is needed to reduce the experimental error in finding out the stretch. Commercially available video extensometers of this type are manufactured by Instron Corp. Canton Mass. Such devices are rather big and can best measure flat, well-marked specimens. For in-vivo measurements, however, the curvature of the skin due to natural body contour interferes with the exact measurements of the distances between the marked points.
Mechanical properties of skin, such as elastic and viscoelastic, have been investigated experimentally and theoretical models have been proposed.
Wounds created by accidents or surgical procedures involve trauma to both the skin and the underlying tissue. Wound healing proceeds through several stages, including an initial inflammatory response and cell proliferation and migration. The inflammatory response at the wound site involves leukocyte infiltration and the local release of chemical mediators of inflammatory such as histamine, platelet activating factor, leukotrienes and endothelin-1 by leukocytes and endothelial cells. It has been well established that cellular production and release of inflammatory mediators are altered by mechanical stress. Physical properties of skin play a dominant role in closure of wounds. When the skin is stretched during suturing, stresses are produced. Skin tension is of particular importance in wound healing. High tension across a sutured wound is likely to produce a stretched hypertropic scar at that site. Better scars are produced when the axis of a wound are placed parallel to Langer's lines compared to the case when the axis cross the lines. Langer's lines described as the natural tension lines exist in the skin. A circular incision on the skin becomes an elliptical shape where the long axis of the ellipse lies parallel to the Langer's lines. Dehiscence, ischemia or necrosis may be expected in regions of high stresses through compromise of circulation in the subdermal vascular plexus. Blood flow is inversely proportional to wound closure tension as observed in animal studies. Therefore, it is worthwhile to determine stresses and/or deformations accurately for a given pattern of suturing of the wound. A preferred suturing pattern and wound geometry should produce low average tensile stresses with the lowest possible stress gradient at the critical points of the wound edges.
The principles and the techniques of tissue expansion procedure have been widely used in plastic surgery and many types of tissue expanders are available for the skin. The implant, approximately the size and shape of the donar area, is placed under the skin and subcutaneous tissue. If the wound tension is minimal at the time of tissue expander placement, a moderate volume of saline may be introduced without delay. This initial saline lubricates the interior of the implant and may reduce the likelihood of fold flaw erosion. The immediate introduction of saline may also place enough tension on the margins of the wound to lessen seroma and hematoma formation. Only sufficient saline should be placed at the time of implant placement to fill the dissection space without placing any undue tension on the suture line. Inflations are generally begun one or two weeks after implant placement, although inflation schedules must be individualized to the nature and anatomic location of the deformity. For practical reasons, most prostheses are inflated at weekly intervals, but highly accelerated inflation schedules have been used. Each inflation proceeds to a point of patient discomfort or blanching of the skin overlying the implant. In anesthetic regions, such as in the treatment of pressure sores, objective changes in flap vascularity should be evaluated. Although a variety of pressure transducers, oxygen tension monitors, and other types of perfusion monitoring devices are available as adjuncts to inflation of tissue expanders, objective inspection and patient response are usually used to judge appropriate implant inflation.
Skin reflectance spectroscopy has been used for measuring physiological variations in skin color and erythems. The oxyhemoglobin (amount of blood) and melanin pigment play important role in the reflection properties of skin.
Existing technologies of the related areas can be summarized in the following way: Laser speckle and holography have been used to measure small displacements of skin. However this interferometric technique has some severe technical difficulties for in-vivo use due to motion of the subject. Also a laser diffraction apparatus has been designed to measure the sarcomere length of muscle in-vivo, which is useful for tendon transfer procedures. The diffraction patterns of the striated muscle fibril due to Z dark lines have been utilized in this technique. This method will not work for skin or other soft tissues because of the lack of fine natural grating lines for laser diffraction. Soft tissue stretch is commonly measured by marking the tissue on its surface using a pen or colored tape and recording the changes in marker position with stretch. The recording device can be photographic film or video. In this method a large distance between the marked lines (Long gage length) on soft tissue is needed to reduce the experimental error in measuring the stretch. Commercially available video extensometers are rather big and can best measure flat, well-marked specimens. For in-vivo measurements, however, the curvature of the skin and other soft tissues due to natural body contours interfere with the exact measurements of the distances between the marked points.
Optical properties of biological tissues and skin are related to their structure and their chemical composition. When a beam of light reaches the skin surface, part of it will be reflected by the surface directly, while the rest will be refracted and transmitted into the skin. The direct reflection of light by a surface, where the angle of reflected light is equal of incident light, is called specular reflection, and is related only to the refractive index change between the air and reflecting medium. By nature, the reflection of light from soft tissue is diffuse rather than specular. The intensity and angular distribution of diffusion reflection is determined by transmission and scattering properties of the tissue. After diffuse reflection from the top layer of the tissue, a portion of light transmitted through the top layer and into the tissue will be scattered and absorbed by the tissue. After multiple scattering and reflection from the different layers of soft tissue, some of the transmitted light will re-emerge through the air-stratum interface into the air as part of the total reflected light intensity.
The real time measurement of the skin stretch and estimation of stresses in skin is an important problem in plastic surgery. Excessive tensile stresses delay wound healing and cause scar tissue and granulation.